1. Field of the Invention
This invention relates to improved catalysts for the selective dehydrogenation of dialkyl aromatic hydrocarbons to produce alkyl vinyl aromatic hydrocarbons, more particularly to catalysts for the production of para-methylstyrene (PMS) via the dehydrogenation of para-ethyltoluene (PET).
2. The Prior Art
The vinyl benzenes play a particularly important role in the preparation of synthetic plastics and resins. The polymerization of styrenes, for example, to produce polystyrene resins is well known.
Styrene and styrene derivatives are typically produced from ethylbenzene materials by dehydrogenation over solid catalysts in the presence of co-fed steam, and at temperatures ranging from 500.degree. to 700.degree. C. The catalysts found to be the most effective for this process are those which are based on potassium oxide (carbonate) promoted, chromium oxide stabilized, iron oxide material. Catalysts of this type are said to be self-regenerative inasmuch as, in addition to their effectivness in promoting dehydrogenation, they also promote the water gas reaction in the presence of the steam co-feed, to thereby remove coke which would otherwise build up on and deactivate the catalyst. The lifetime of such self-regenerative catalysts is thus determined by the effectivness of the catalyst in maintaining its activity for conversion of ethylbenzene materials such as para-ethyltoluene for any given steam/hydrocarbon ratio in the feed. Catalysts of this type which can maintain such activity at generally lower steam/hydrocarbon ratios are, of course, more economically desirable.
Another problem which can diminish the economical production of certain styrene derivatives such as p-methylstyrene via the steam dehydrogenation of p-ethyltoluene is the problem of "popcorn" polymer formation. Popcorn polymers are those solid polymeric materials which are popcorn-like in appearance and which can form and build up at the relatively cooler exit port of the dehydrogenation reaction reactor vessel and in the condenser and other parts of the cooling train used to recover dehydrogenation products. Popcorn polymer formation does not generally occur when ethylbenzene is dehydrogenated but can be a significant problem during dehydrogenation of para-ethyltoluene. While various inhibitors can be added to the dehydrogenation reactor effluent to minimize popcorn polymer formation, the eventual buildup of such materials can necessitate shutdown of the dehydrogenation reaction equipment to remove the accummulated popcorn polymer. Obviously, dehydrogenation catalysts which reduce the inherent tendency of popcorn polymers to form in the dehydrogenation reaction effluent would be economically attractive for commercial scale production of styrenic materials.
Yet another problem which can occur when styrene derivatives such as p-methylstyrene are prepared from ethyltoluene materials by dehydrogenation over catalysts of the type herein involved in the phenomenon of aromatic ring loss. The catalyst employed to effect conversion of the ethyltoluene materials can to some extent also promote cracking of the ring structures of the aromatic reactants and products. Obviously it is desirable to identify those catalytic materials which minimize the amount of aromatic ring loss during the dehydrogenation reaction while maintaining high conversion of ethyltoluene to p-methylstyrene.
In view of the foregoing considerations, there is clearly a continuing need to formulate steam regenerative dehydrogenation catalysts suitable for promoting production of particular styrenic materials from substituted ethylbenzene materials, e.g., selective production of p-methylstyrene from p-ethyltoluene, with improved catalyst lifetimes, especially at lower steam/hydrocarbon ratios in the reactor feed, with minimized propensity of the reaction effluent to form popcorn polymer, and with minimized loss of aromatic ring structure during the reaction.
Accordingly, it is an object of the present invention to provide an improved iron oxide based, steam-regenerative dehydrogenation catalyst especially useful for the dehydrogenation of para-ethyltoluene to selectively produce para-methylstyrene.
It is a further object of the present invention to provide such dehydrogenation catalysts having extended catalyst lifetime, with acceptable activity for low ring loss conversion of p-ethyltoluene and with desirably high selectivity to production of p-methylstyrene, even at low steam/hydrocarbon ratios in the charge to the reaction zone.
It is a further object of the present invention to provide such a catalyst which produces a p-methylstyrene-containing dehydrogenation reaction effluent having minimized tendency to form solid "popcorn" polymer.
It is a further object of the present invention to provide a low aromatic ring loss para-ethyltoluene dehydrogenation process employing a catalyst having extended lifetime with minimal tendency of the dehydrogenation reaction effluent to form popcorn polymer.
These and other objectives can be achieved by means of the invention described and claimed herein.